The invention relates to an antenna for transmitting and/or receiving linearly polarized signals.
In the field of telecommunications, a particular carrier frequency is often used to transmit signals having different polarizations. This provides two channels transmitting different information on the same carrier. If the polarization is circular, one channel is allocated to right-hand circularly polarized signals and the other channel is allocated the signals having a polarization in the opposite direction, i.e. left-hand circular polarization. If the polarization is linear, one channel transmits with "parallel" (or "horizontal") polarization and the other channel transmits signals with a perpendicular (or "vertical") polarization.
Antennas, and in particular ground antennas, are generally designed to receive and/or to transmit signals in the same direction and with a particular polarization. Thus an antenna must both be pointed correctly at the source of transmission and, in the case of linear polarization, it must also be oriented about its axis to maximize reception of the linearly polarized signal.
When an antenna is designed to receive signals from a geostationary satellite, for example, the axis of the antenna must be pointed towards the satellite and its orientation about its axis must maximize the received signal.
An antenna for linearly polarized signals must be set up with particular care. In particular, if the setup is defective, the antenna transmits both with the required polarization and with the perpendicular polarization, which causes interference signals to be transmitted, which signals can then be received by other receivers designed for the perpendicular polarization. Standards generally require an antenna transmitting with linear polarization in a particular direction to be able to transmit with the perpendicular linear polarization only at a level that does not exceed some limit; this limit level is reached if the antenna's angular orientation departs only approximately 1.degree. from its optimum orientation.
Until now, the polarization of the antenna has been adjusted manually.
It has been found that the result of such adjustment is not always satisfactory. Furthermore, it can vary with time.
Also, the adjustment of the antenna is verified only by confirming that the received signal has the expected qualities. When a television signal is received, for example, the adjustment is deemed to be correct when the quality of the picture is satisfactory. This type of verification is sometimes insufficient, however, especially in the case of a transmit antenna.